The present invention relates to a process for treating a metal surface, particularly in order to improve its adhesion properties.
Our published patent application No. WO96123037 describes a process for treating a metal surface which is aluminium or an aluminium alloy in order to improve its adhesion properties which comprises treating the clean metal surface with an organosilane and exposing the surface to a laser.
We have now surprisingly found that the organosilane and laser treatment is also suitable in pretreatment of other metal surfaces prior to bonding or other processing where adhesion properties of the surfaces are important and also in the pretreatment of an aluminium or aluminium alloy surface which is not clean.
Accordingly the present invention provides a process for treating a metallic surface, other than a clean aluminium or aluminium alloy surface, which comprises treating the surface with an organosilane and exposing the surface to a laser.
The metallic surface may be of any metal. Examples of suitable metals include steel, e.g. stainless steel, iron in ferrite form, titanium, magnesium, copper, gold, nickel or chromium or an alloy of said metal. The metallic surface may also be a non-clean aluminium or aluminium alloy surface.
The metallic surface may be treated with the organosilane and the laser in either order. Preferably, however, an organosilane is first applied to the metallic surface and the coated surface is exposed to a laser.
It is also possible to expose the surface to a laser, apply an organosilane, and then expose the surface again to a laser.
The metallic surface may be clean; for example, it may be degreased, generally using any standard method such as wiping with a solvent such as acetone, vapour degreasing, dipping with or without ultrasonic treatment or by using alkaline degreasing agents. The process of the invention can also be used to treat the metallic surface when it is not clean, for example when it is oily or when it is aged.
The organosilane may be a compound of formula I
RnSi(OR1)mxe2x80x83xe2x80x83(I)
where R is an organic group which may be reactive or non reactive, R1 is alkyl, alkoxyalkyl or acyl, n is 1 or 2 and m is 2 or 3 such that n+m=4. Preferably n is 1 and m is 3.
Examples of organic groups R include alkyl, phenyl, vinyl, acrylatoalkyl, glycidyloxyalkyl and the like, where xe2x80x9calkylxe2x80x9d groups have 1 to 4 carbon atoms.
When R1 is alkyl, it is preferably lower alkyl with 1 to 4 carbon atoms and most preferably methyl or ethyl. When R1 is alkoxyalkyl, each alkyl moiety preferably has 1 to 4 carbon atoms. Most preferably as alkoxyalkyl R1 is methoxyethyl. When R1 is acyl it preferably has 2 to 4 carbon atoms, most preferably acetyl.
Specific examples of suitable silanes are:
CH2xe2x95x90C(CH3)COO(CH2)3Si(OCH3)3
CH3Si(OCH3)3 
C6H5Si(OCH3)3 
CH3(CH2)2Si(OCH3)3 
HS(CH2)3Si(OCH3)3 
CH2xe2x95x90CHSi(OOCCH3)3 
CH2xe2x95x90CHSi(OCH2CH3)3 
CH2xe2x95x90CHSi(OCH3)3 
Cl6(CH2)3Si(OCH3)3 and
CH2xe2x95x90CHSi(OCH2CH2OCH3)3 
The silane may be used in solution in water or an organic solvent.
If water is used as the solvent and the silane is difficult to dissolve, a small amount of a nonionic wetting agent may be added to the water before the silane addition. Alternatively, the silane may be used as an emulsion.
Suitable organic solvents include alcohols, esters, ethers, ketones and chlorinated hydrocarbons. Preferred alcohols are alkanols with 1 to 10 carbon atoms such as methanol, ethanol, propanol, hexanol and decanol. Preferred esters are C1-C4 alkyl esters of C1-C4 aliphatic carboxylic acids such as methyl acetate, ethyl acetate, butyl acetate and methyl propionate. Preferred ethers are dialkyl ethers such as diethyl ether, butoxyethanol and cyclic ethers such as tetrahydrofuran. Preferred chlorinated hydrocarbons are dichloromethane, 1,2-dichloroethane, and trichloromethane. Preferred ketones are lower aliphatic ketones such as acetone and methyl ethyl ketone. Mixtures of these solvents may be used if desired. The most preferred solvents are polar solvents such as ketones, especially acetone. Water and/or a carboxylic acid may also be added to suitable organic solvents as part of the solution.
The solution may contain any concentration of silane preferably from 1 to 10% by weight of the silane based on the total weight of the solution.
The organosilane solution may be applied by any suitable method, for example, wiping, brushing or spraying on to the areas to be treated.
Degreasing, when desired, of the metal surface, the application of organosilane solution and the laser treatment may all be carried out with automatic equipment such as robots.
Any suitable laser may be used, e.g. at 400 nJ/pulse. Suitable lasers include, for example excimer lasers and Q-switched Nd:YAG lasers. Others are well known in the literature.
For high treatment speeds and not to damage the metal surface through unacceptable high energy concentrations, good results can be obtained using an unfocussed laser.
The actual power level needed to avoid damaging the metal surface depends on the actual surface being treated and on the specific laser used. This can be readily determined by simple experiment.
After treatment of the metallic surface by the process of the invention, the surface is ready for bonding or other processing where the adhesion properties of the surface are important, for example coating or encapsulating. The bonding may be to another surface by means of an adhesive or, e.g. by applying a coating to the surface. When bonding to another surface, that other surface may be metallic or non metallic. If it is metallic, it too may be pretreated by the same process as described above if desired.
When the treated surface is bonded to another surface, this may be achieved using any adhesive such as 1-component or 2-component epoxy or polyurethane adhesives. Preferably the adhesive used is one which will react with the organosilane used in the process of the invention.
The process of the invention provides excellent joint performance, a fast treatment, a clean process, ecological advantages over conventional xe2x80x9cwetxe2x80x9d processes and sandblasting processes, allows for the use of a wide range of adhesives and makes local treatment of the areas to be bonded possible. A marked improvement in adhesion properties of metal surfaces can be obtained by the process of the invention.
The invention is illustrated by the following Examples.